High cleanliness spring steel and high cleanliness spring excellent in fatigue properties

ABSTRACT

High cleanliness spring steel useful in manufacturing a spring with SiO 2 -based inclusions being extremely controlled and excellent in fatigue properties is provided. 
     High cleanliness spring steel which is steel containing;
         C: 1.2% (means mass %, hereafter the same with respect to the component) or below (not inclusive of 0%),   Si: 1.2-4%,   Mn: 0.1-2.0%,   Al: 0.01% or below (not inclusive of 0%),
 
and the balance comprising iron with inevitable impurities, wherein;
 
the total of oxide-based inclusions of 4 or above of L (the large diameter of an inclusion)/D (the short diameter of an inclusion) and 25 μm or above of D and oxide-based inclusions of less than 4 L/D and 25 μm or above of L, in the oxide-based inclusions of 25 mass % or above of oxygen concentration and 70% (means mass %, hereafter the same with respect to inclusions) or above of SiO 2  content when Al 2 O 3  +MgO+CaO+SiO 2 +MnO=100% is presumed, out of inclusions in the steel, is 20 nos./500 g or below.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of prior U.S. patentapplication Ser. No. 12/301,544, the disclosure of which is incorporatedby reference in its entirety. U.S. Ser. No. 12/301,544 is a NationalStage of PCT/JP07/60538 filed on May 23, 2007 which claims the benefitof priority under 35 U.S.C. §119 from Japanese Patent Application No.2006-160796, filed Jun. 9, 2006, and Japanese Patent Application No.2006-161240, filed Jun. 9, 2006 the disclosures of which areincorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to high cleanliness spring steel excellentin fatigue properties and a high cleanliness spring excellent in fatigueproperties, and relates to high cleanliness spring steel wherein hardmetallic inclusions with extremely low ductility are reduced and fatigueproperties are improved, and a high cleanliness spring excellent infatigue properties obtained using the steel. The spring obtained fromthis high cleanliness spring steel is useful as a valve spring and aclutch spring for an automobile engine, and a brake spring whereinextremely high fatigue properties are required.

2. Background Art

In recent years, in the field of automobiles for example, as weightreduction and output enhancement are more highly required, a high stressdesign is directed also in a valve spring and a suspension spring usedfor an engine, a suspension and the like, and a clutch spring and thelike. Therefore, for these springs, ones which are excellent in fatigueresistance property and setting resistance property are stronglyrequired to cope with increase in load stress. In particular, withrespect to a valve spring, the requirement for increasing fatiguestrength is very strong, and even SWOSC-V (JIS G 3566), which is said tobe high in fatigue strength among conventional steel, is hard to copewith.

In spring steel material wherein high fatigue strength is required, itis necessary to reduce hard nonmetallic inclusions present in the steelas much as possible. From such a viewpoint, with respect to the steelused for the usage described above, it is common that high cleanlinesssteel wherein presence of nonmetallic inclusions described above isdecreased as much as possible is used. Because the risk of wire breakageand fatigue breakage due to nonmetallic inclusions increases as highstrengthening of material progresses, the requirement for reduction andminiaturization of the nonmetallic inclusions described above whichbecomes its main cause has become greatly severe.

From the viewpoint of realizing reduction and miniaturization of hardnonmetallic inclusions in steel, a variety of technologies have beenproposed so far. For example in the Patent Document 1, it is disclosedthat, in valve spring steel, if controlled to CaO-Al₂O₃-SiO₂three-component based inclusions whose melting point is lower thanapproximately 1,400-1,500 DEG C., they do not become the start point offatigue failure and fatigue properties improve.

Also, in the Patent Document 1, high cleanliness steel, whose coldworkability and fatigue property are improved by stipulating the averagecomposition of nonmetallic inclusions so that the nonmetallic inclusionsare well stretched in hot rolling and are crushed and finely dispersedin a cold rolling or wire drawing process, is realized. Further, in thePatent Document 2 also, the composition of nonmetallic inclusions whichare easily stretched and refined in hot rolling, cold rolling anddrawing is stipulated.

On the other hand, in the Patent Document 3, a technology is disclosedwherein the melting point of an oxide-based inclusion is stipulated as1,500 DEG C. or below to make it an inclusion having a low-meltingcomposition easily stretched in hot and cold rolling.

Further, in the Patent Document 4, as an inclusion having a low-meltingcomposition easily stretched in hot rolling and cold rolling,Al₂O₃-SiO₂-MnO based one containing MgO and/or CaO is stipulated.

Non-patent Document 1: “182^(nd) and 183^(rd) Nishiyama MemorialTechnical Lecture”, edited by The Iron and Steel Institute of Japan,P.131-134.

Patent Document 1: Japanese Unexamined Patent Application PublicationNo. S62-99436

Patent Document 2: Japanese Unexamined Patent Application PublicationNo. S62-99437

Patent Document 3: Japanese Unexamined Patent Application PublicationNo. H5-320827

Patent Document 4: Japanese Unexamined Patent Application PublicationNo. S63-140068

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In a variety of conventional technologies proposed until now, it wascommon to realize excellent fatigue strength by controlling the averagecomposition of inclusions. In the meantime, in recent years, followingproperties (I) and (II) are required for a valve spring.

(I) Because there is a tendency of securing the space below a bonnet toensure collision safety for pedestrian protection, reduction of engineheight, that is, reduction of spring height, is required.(II) For improving fuel economy, weight reduction of a spring isrequired.

As described above, although further high strengthening of spring steelis required for realizing reduction of spring height and reduction ofweight, it is not possible to cope with the requirement of further highstrengthening merely by conventional technologies. Its reason is thatthere is a problem that control of inclusions of very high level hasbeen already performed so far, inclusions inducing breakage exist veryrarely only, and it is difficult to reduce further. Also, it has beenknown that hard crystal-based inclusions would be harmful, however,there has been no knowledge on distinction of particularly harmful oneand not among hard crystal-based inclusions.

Further, in a variety of conventional technologies proposed until now,main point is to control the inclusion composition to a low meltingpoint region and to aim to effect refining, however, if the compositionregion is one which includes SiO₂, SiO₂ may theoretically be generatedby phase separation. Particularly, because valve spring steel is heatedbefore rolling after forging, occurring of generation of SiO₂ by phaseseparation becomes highly probable at this time. SiO₂ thus generated ishard, is hardly deformed in rolling, and easily remains in the finalproduct. SiO₂ remaining in the final product may become the cause ofbreakage in a wire rod even it is in extremely small numbers, andrealization of high cleanliness steel capable of coping with requirementin recent years for further enhancement of cleanliness may not becomepossible.

The present invention has been developed under such circumstances, andits purpose is to provide spring steel useful in obtaining a springexcellent in fatigue properties and a spring excellent in fatigueproperties.

Means to Solve the Problems

The purpose of the present invention described above can be achieved byfollowing high cleanliness spring steel in relation with the presentinvention. That means, a first high cleanliness spring steel in relationwith the present invention is: steel containing;

-   -   C: 1.2% (means mass %, hereafter the same with respect to the        component) or below (not inclusive of 0%),    -   Si: 1.2-4%,    -   Mn: 0.1-2.0%,    -   Al: 0.01% or below (not inclusive of 0%), and the balance        comprising iron with inevitable impurities,        wherein;        total of oxide-based inclusions of 4 or above of L (the large        diameter of an inclusion)/D (the short diameter of an inclusion)        and 25 μm or above of D and oxide-based inclusions of less than        4 L/D and 25 μm or above of L, in the oxide-based inclusions of        25 mass % or above of oxygen concentration and 70% (means mass        %, hereafter the same with respect to inclusions) or above of        SiO₂ content when Al₂O₃+MgO₊CaO+SiO₂+MnO=100% is presumed, out        of inclusions in the steel, is 20 nos./500 g or below.

The first high cleanliness spring steel described above may furthercontain, as other elements:

-   -   (a) one or more kinds selected from a group consist of;        -   Cr: 3% or below (not inclusive of 0%),        -   Mo: 0.5% or below (not inclusive of 0%),        -   W: 0.5% or below (not inclusive of 0%), and        -   Co: 0.5% or below (not inclusive of 0%),    -   (b) one or more kinds selected from a group consist of;        -   V: 0.5% or below (not inclusive of 0%),        -   Nb: 0.1% or below (not inclusive of 0%), and        -   Ti: 0.1% or below (not inclusive of 0%),    -   (c) Cu: 0.1% or below (not inclusive of 0%) and/or Ni: 0.5% or        below (not inclusive of 0%),    -   (d) REM by 0.1-50 ppm,    -   (e) total of alkali metal elements and/or alkaline earth metal        elements by 0.1-50 ppm.

Further, a second high cleanliness spring steel in relation with thepresent invention is characterized that: oxide-based inclusions with awidth of 3 μm or above which are present in the surface layer side fromthe surface of a wire rod to the depth of ¼ of the diameter satisfyinequality (1) below, further, the concentration of MgO: 5 mass % orbelow (inclusive of 0 mass %) and the concentration of MnO: 10 mass % orbelow (inclusive of 0 mass %), also, when each concentration [Cn] , [An]and [Sn] of CaO, Al₂O₃ and SiO₂ in the oxide-based inclusions areexpressed respectively as per the equations (2)-(4) below, they satisfythe relation of the equations (5) and (6) below, and when these CaOconcentration [Cn], Al₂O₃ concentration [An] and SiO₂ concentration [Sn]are expressed by a CaO—Al₂O₃ SiO₂ three component system phase diagram,the melting point of the oxide-based inclusions is 1,500 DEG C. orbelow. Here, “width of inclusion” means the diameter of an inclusion inthe direction perpendicular to the axial direction when observed by asection including the axis line of the wire rod.

CaO+Al₂O₃+SiO₂>80 (mass %)   (1)

[Cn] (mass %)=[(CaO)/(CaO+Al₂O₃+SiO₂)]×100   (2)

[An] (mass %)=[(Al₂O₃)/(CaO+Al₂O₃+SiO₂)]×100   (3)

[Sn] (mass %)=[(SiO₂)/(CaO+Al₂O₃+SiO₂)]×100   (4)

[An]+4.29 [Sn]≦221.9 (mass %)   (5)

[An]≦30 (mass %)   (6)

Also, in the spring steel described above, MgO concentration and MnOconcentration, as well as CaO, Al₂O₃, SiO₂ in the inequality (1)represent mass % against total inclusion particles.

Although the chemical componential composition of the second highcleanliness spring steel described above is not particularly limited asfar as it is suitable composition for spring steel, as a preferable one,steel material respectively containing C: 1.2 mass % or below (notinclusive of 0%), Si: 1.2-4 mass %, Mn: 0.1-2.0 mass %, Al: 0.01 mass %or below (not inclusive of 0%) can be exemplarily cited. Also, this highcleanliness spring steel may further contain one or more kinds selectedfrom a group consist of Cr, Ni, V, Nb, Mo, W, Cu, Ti, Li, Na, K and rareearth elements. Preferable contents in containing them are, Cr: 0.5-3mass %, Ni: 0.5 mass % or below, V: 0.5 mass % or below, Nb: 0.1 mass %or below, Mo: 0.5 mass % or below, W: 0.5 mass % or below, Cu: 0.1 mass% or below, Ti: 0.1 mass % or below, Li: 0.0005 mass % or below, Na:0.0010 mass % or below, K: 0.0010 mass % or below, and rare earthelements: 0.0010 mass % or below. Also, the concentration of thesecomponents represents the concentration in the steel.

Furthermore, the present invention also includes high cleanlinesssprings excellent in fatigue properties obtained using the first springsteel and the second spring steel described above.

Effects of the Invention

In accordance with the first high cleanliness spring steel in relationwith the present invention, realization of high strengthening ispossible by performing severe drawing work because SiO₂-based inclusionsare extremely controlled, and high cleanliness spring steel capable ofmanufacturing springs excellent in fatigue properties easily and springsexcellent in fatigue properties obtained using the steel can berealized.

Also, in accordance with the second high cleanliness spring steel inrelation with the present invention, high cleanliness spring steel thatobtains springs excellent in fatigue properties can be realized bymaking entire inclusions with low melting point and easy to be deformed,and hard to generate SiO₂ even if phase separation occurs in heatingbefore hot rolling and during hot rolling.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] A graph showing the investigation results on the effect of thenumber of SiO₂-based inclusions (nos./500 g steel) on fatigue properties(breakage ratio obtained by a method described in Example).

[FIG. 2] An explanatory drawing showing an abstraction procedure ofSiO₂-based inclusions in Example.

[FIG. 3] A CaO—Al₂O₃—SiO₂ three component system phase diagram showing aregion where SiO₂ is generated by phase separation in heating before hotrolling and during hot rolling.

[FIG. 4] A CaO—Al₂O₃—SiO₂ three component system phase diagram showingthe composition of inclusions stipulated by the present invention.

DESCRIPTION OF THE REFERENCE NUMERALS

1. Beaker

2. Aqueous solution of nitric acid

3. Sample

4. Heater

5. Membrane filter

6. Filter stand

7. Funnel

8. Flask

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Best Mode for CarryingOut the Invention The First Embodiment

The present inventors made investigations from a variety of angles toobtain spring steel suitable to manufacturing of springs that exertexcellent fatigue properties. As a result, it was found out that, amonghard crystal-based inclusions (to be exact, oxide-based inclusions with25 mass % or above oxygen concentration), SiO₂-based inclusions inparticular were harmful. More specifically, it was found out that,because the composite inclusions which were inclusions of SiO₂ or ofhigh SiO₂ percentage and were inclusions of 4 or above of L (the largediameter of an inclusion)/D (the short diameter of an inclusion) and 25μm or above of D or inclusions of less than 4 of L/D and 25 μm or aboveof L extremely lowered fatigue properties even though the density wasextremely low, it was necessary to strictly control the number of theinclusions described above, and the present invention was completed.

Further more specifically, as will be shown in Examples described below,the total of inclusions described below obtained by abstraction fromsteel should be made 20 nos./500 g or below.

-   -   Inclusions with 70% or above of SiO₂ content (presuming        Al₂O₃+MgO+CaO+SiO₂+MnO=100%, hereafter the same. Also, % means        mass %, hereafter the same with respect to inclusions) and with        4 or above of L (large diameter of inclusion)/D (short diameter        of inclusion) and 25 μm or above of D, and    -   Inclusions with 70% or above of SiO₂ content (presuming        Al₂O₃+MgO+CaO+SiO₂+MnO=100%, hereafter the same) and with less        than 4 of L/D and 25 μm or above of L.        (Hereafter, these inclusions abstracted may be collectively        referred to as “SiO₂-based inclusions”.)

FIG. 1 is a graph showing the investigation results on the effect of thenumber of the SiO₂-based inclusions described above on fatigueproperties (breakage ratio obtained by a method described in Exampledescribed below). As shown in FIG. 1, it is known that the breakageratio becomes extremely small by controlling the SiO₂-based inclusionsto 20 nos./500 g or below. Preferably, if the SiO₂-based inclusions arecontrolled to 12 nos./500 g or below, fatigue properties can be furtherimproved.

In order to obtain the steel with SiO₂-based inclusions being controlledas described above, a method, for example, can be cited wherein, invacuum induction melting, carbon deoxidation in vacuum as following isperformed, and oxygen level is lowered sufficiently (3 ppm or below).That is, inside the furnace is evacuated (60 Torr or below) and C isfinely dividedly added until the target C concentration is reached. Freeoxygen is measured by a zirconia oxygen sensor, if it is not the targetlevel or below (3 ppm or below), inside the furnace is evacuated (60Torr or below) again (at this time, partial CO pressure inside thefurnace lowers, therefore, C and O in molten steel react andconcentration of C in molten steel also lowers), and C is finelydividedly added until the target C concentration is reached.

The spring steel in relation with the present embodiment is to satisfythe following fundamental components. Firstly, C is an element useful insecuring high strength, and it is preferable to make C content 0.2%(means mass %, hereafter the same with respect to the component) orabove in order that the effect is sufficiently exerted. More preferablyit is 0.3% or above, and further more preferably it is 0.4% or above.However, if C content becomes excessive, steel becomes brittle which isnot practical, therefore it is controlled to 1.2% or below.

In the spring steel in relation with the present embodiment, the steelof Si: 1.2% or above is made an object. However, if Si is excessivelycontained, steel material becomes liable to be embrittled, therefore Siis controlled to 4% or below.

In order to control the composition of inclusions, Mn, which isdeoxidation component, is made contain by 0.1% or above. However, ifthis component is excessively contained, steel material becomes liableto be embrittled, therefore Mn content is controlled to 2.0% or below.

Al is an element useful in control of inclusions and is required atapproximately 0.0001% as the total Al. However, if the total Al contentbecomes much, Al₂O₃ concentration in inclusions becomes high and it ispossible that coarse Al₂O₃ which becomes the cause of wire breakage isgenerated, therefore it is controlled to 0.01% or below.

The elements contained in the spring steel in relation with the presentinvention are as described above, and the balance is iron and inevitableimpurities. For the inevitable impurities, mixing in of elements broughtin by the situation of raw material, manufacturing materials,manufacturing equipment and the like is allowable. Also, it is effectiveto further improve properties by positively containing elementsdescribed below.

{One or more kinds selected from a group consist of Cr: 3% or below (notinclusive of 0%), Mo: 0.5% or below (not inclusive of 0%), W: 0.5% orbelow (not inclusive of 0%), and Co: 0.5% or below (not inclusive of0%)}

These elements are the elements effective in improving softeningresistance property, and in order to make the effect exert, it ispreferable to contain by 0.5% or above for Cr, 0.05% or above for Mo,0.05% or above for W, and 0.01% or above for Co. However, if theseelements are excessive, quenching property becomes too high and breakagebecomes liable to occur during working, therefore it is preferable tocontrol to 3% or below for Cr, 0.5% or below for Mo, 0.5% or below forW, and 0.5% or below for Co.

{One or more kinds selected from a group consist of V: 0.5% or below(not inclusive of 0%), Nb: 0.1% or below (not inclusive of 0%), and Ti:0.1% or below (not inclusive of 0%)}

These elements are the elements effective in grain refinement, and inorder to make the effect exert, it is preferable to contain by 0.01% orabove for V, 0.01% or above for Nb, and 0.01% or above for Ti. However,if these elements are excessive, coarse nitride is generated and fatiguestrength is lowered. Therefore, it is preferable to control to 0.5% orbelow for V, 0.1% or below for Nb, and 0.1% or below for Ti.

{Cu: 0.1% or below (not inclusive of 0%) and/or Ni: 0.5% or below (notinclusive of 0%)}

These elements are the elements effective in inhibiting low temperatureembrittlement, and in order to make the effect exert, it is preferableto contain by 0.05% or above for Ni, and 0.01% or above for Cu.

However, even if these elements are contained excessively, its effectsaturates only, therefore it is preferable to make Ni at 0.5% or below,and Cu at 0.1% or below, from the economical viewpoint.

{REM: 0.1-50 ppm}

REM (rare earth metal; Ce, La and the like) has an action of furthersoftening nonmetallic inclusions in steel. To make the effect exert, itis preferable to contain 0.1 ppm or above. However, even if the elementis added excessively, the effect saturates only, therefore it ispreferable to make it 50 ppm or below.

{Alkali metal elements and/or alkaline earth metal elements: 0.1-50 ppmin total}

Alkali metal elements (Li, Na, K, Rb, Cs) and alkaline earth metalelements (Mg, Ca, Ba, Sr) have an action of further softeningnonmetallic inclusions in steel. To make the effect exert, it ispreferable to contain 0.1 ppm or above in total. However, even if theelements are added excessively, the effect saturates only, therefore itis preferable to make it 50 ppm or below in total.

As described above, the spring steel in relation with the firstembodiment of the present invention is excellent in drawing workabilitybecause SiO₂-based inclusions are extremely controlled, and can secureexcellent fatigue properties. Consequently, the spring steel in relationwith this embodiment is useful in manufacturing springs used, forexample, in the automobile field, the industrial machinery field and thelike. In particular, it is best suited to manufacturing springs used fora restoring mechanism of machinery such as a valve spring and a clutchspring for an automobile engine, a brake spring, a suspension spring fora suspension and the like which require extremely high fatigueproperties, and the like.

The Second Embodiment

Next, an embodiment in relation with the second spring steel of thepresent invention will be described.

In a wire rod with a large deformation ratio during hot rolling, it isknown that refinement of inclusions by stretchingly tearing during hotrolling is useful. Conventionally, because hard inclusions weredisliked, control to low melting point composition (the Patent Document3) and control to a certain composition region (the Patent Document 4)were performed. However, because the main aim was the control to lowmelting point composition, the composition was such that SiO₂ would begenerated theoretically, and SiO₂ was generated rarely.

Under such circumstance, the present inventors studied from a variety ofangles on composition and a form of respective inclusion for improvingfatigue resistance property of springs considering also the change inthe form of inclusions by heating after solidification and by hotrolling. As a result, it was known that, by controlling to thecomposition wherein SiO₂ would not be generated theoretically,generation of SiO₂ could be inhibited remarkably regardless of rollingcondition. In other words, as will be known from FIG. 4 exhibited below,even within the range of the low melting point, the melting point in theside where SiO₂ concentration is high is occasionally lower comparedwith that in the side where SiO₂ concentration is low. Consequently,controlling to the lower side of SiO₂ concentration was not common.Then, the present inventors found out that harmful SiO₂ could beremarkably controlled by performing control of the composition to apredetermined region and that tearing was possible during hot rollingeven in the side where SiO₂ concentration was low, and the presentinvention was completed.

The working effects by the constitution of this embodiment will bedescribed successively. In the spring steel in relation with thisembodiment, it is necessary that the oxide-based inclusions with thewidth: 3 μm or above which are present in the surface layer side fromthe surface of a wire rod to the depth of 1/4 of the diameter satisfyinequality (1) below, and the concentration of MgO is 5 mass % or below(inclusive of 0 mass %) and the concentration of MnO is 10 mass % orbelow (inclusive of 0 mass %).

CaO+Al₂O₃+SiO₂>80 (mass %)   (1)

The reason the inclusions which are made the object here are specifiedas “the width: 3 μm or above” is that the fine inclusion with the width:less than 3 μm is hard to become the start point of the fatigue failureand has not a conspicuous influence on fatigue strength. Also, thereason the position of the presence of such inclusion is specified as“the surface layer side from the surface of a wire rod to the depth of ¼of the diameter” is that the inclusion present in this positioninfluences fatigue properties most.

In a wire rod, irregular inclusions (oxide of Ti, Cr, and the like, forexample) mixed in inevitably, other than CaO, Al₂O₃, SiO₂, MnO and MgO,are present. If their quantity is small, they do not become a problem,but if their quantity increases, they may possibly become the startpoint of the fatigue failure. From such a viewpoint, when thecomposition of inclusions is observed by five elements (CaO, Al₂O₃,SiO₂, MnO and MgO), it is necessary to make [CaO +Al₂O₃ +SiO₂] the mainconstituent (80 mass % or above) and to appropriately control MgOquantity and MnO quantity as well.

Also, if MgO quantity in inclusions becomes excessive, a hard oxide ofMgO-SiO₂, spinel and the like becomes generated, therefore MgO contentin inclusions needs to be made 5 mass % or below.

Further, when composition of inclusions is stably controlled, an oxideof Mn (MnO), which is a weak deoxidation component, becomes 10 mass % orbelow. From such a viewpoint, if MnO content in inclusions exceeds 10mass %, it shows that inclusions are in a non-preferable state,therefore MnO content in inclusions needs to be made 10 mass % or below.

In the spring steel in relation with this embodiment, under thecondition described above, Al₂O₃ quantity when normalized so that threecomponents of CaO, Al₂O₃ and SiO₂ in inclusions become 100% and therelation of Al₂O₃ and SiO₂ and the like need to be strictly stipulatedto satisfy the predetermined region. In other words, when CaO, Al₂O₃ andSiO₂ in oxide-based inclusions are expressed respectively as per theequations (2)-(4) below, they need to be controlled to satisfy therelation of the equations (5) and (6) below.

[Cn] (mass %)=[(CaO)/(CaO+Al₂O₃+SiO₂)]×100   (2)

[An] (mass %)=[(Al₂O₃)/(CaO+Al₂O₃+SiO₂)]×100   (3)

[Sn] (mass %)=[(SiO₂)/(CaO+Al₂O₃+SiO₂)]×100   (4)

[An]+4.29 [Sn]≦221.9 (mass %)   (5)

[An]≦30 (mass %)   (6)

In the spring steel in relation with this embodiment, it is necessarythat SiO₂ is not generated even if phase separation occurs in heatingbefore hot rolling and during hot rolling. For example, in the threecomponent system phase diagram shown in FIG. 3, if the composition ofinclusions is within the composition area indicated by oblique lines,phase separation occurs theoretically and SiO₂ is generated, thereforesuch area needs to be excluded. In the SiO₂ generation region indicatedin FIG. 3, Line A becomes the line connecting CaO-SiO₂ (Wallastonite)and CaO—Al₂O₃—2SiO₂ (Anorthite). In other words, in order that SiO₂ isnot generated even if phase separation occurs in heating before hotrolling and during hot rolling, the region needs to be in the lower sideof low SiO₂ from Line A in FIG. 3 (lower side of Line A in FIG. 3).

The present inventors found out that control to lower SiO₂ side of LineA was effective. The equation (5) described above represents Line A.That means SiO₂ is not generated even if phase separation occurs inheating before hot rolling and during hot rolling, if the relation ofthe equation (5) described above is satisfied.

Also, in the composition of inclusions, if Al₂O₃ quantity whennormalized by three components of CaO, Al₂O₃ and SiO₂ (which is [An]described above) exceeds 30 mass %, hard crystal such as Ghelenite andAnorthite is generated and has a harmful influence on fatigue strength,therefore it is necessary to satisfy the relation of the equation (6).

On the other hand, in the spring steel of the present invention, whenCaO concentration [Cn], Al₂O₃ concentration [An] and SiO₂ concentration[Sn] of the composition of an inclusion are expressed by aAl₂O₃—SiO₂—CaO three component system phase diagram, its melting pointneeds to be 1,500 DEG C. or below. In other words, the reduction ratioof valve spring steel during hot rolling is large and it is expectableto make inclusions refined and non-harmful during hot rolling.Therefore, the control to the composition wherein inclusions are easilydeformed during hot rolling even if its quantity is large is directed.In this embodiment, inclusions need to be controlled to low meltingpoint composition where deformation is easy, and its melting point wasmade 1,500 DEG C. or below. It is preferably 1,400 DEG C. or below.

Area of the composition of inclusions stipulated in this embodiment isshown in FIG. 4 (Al₂O₃—SiO₂—CaO three component system phase diagram).In FIG. 4, Line A is the one satisfying the relation of {[An]+4.29[Sn]=221.9 (mass %)}, Line C is the one satisfying the relation of{[An]=30 (mass %)}, and Line B is the one wherein the melting point ofan inclusion satisfies 1,500 DEG C. (isothermal line). Consequently, inFIG. 4, the area surrounded by Lines A, B, C (shown by hatching in thedrawing) is the area stipulated by the present invention.

In realizing such composition of inclusions as described above, controlof inclusions can be performed easily by using the slag with the samecomposition which is same with the target composition of inclusion.

The steel in relation with this embodiment is assumed of highcleanliness steel useful as a material for spring steel and the like,but its steel kind is not particularly limited. In order to control thecomposition of inclusions, it is preferable to contain Si by 1.2 Mass %or above, Mn by 0.1 mass % or above which are the deoxidationcomponents. However, if these components are contained excessively,steel material becomes liable to be brittle, therefore Si should be 4mass % or below and Si should be 2 mass % or below.

The content of C which is the fundamental component as spring steel ispreferably 1.2 mass % or below. If C content exceeds 1.2 mass %, thesteel material is embrittled and becomes non-practical.

Al is an element useful for control of inclusions also, and is necessaryby approximately 0.1-20 ppm in mass concentration. However, if Alcontent increases, Al₂O₃ concentration in inclusions becomes high andcoarse Al₂O₃ which becomes the cause of wire breakage is possiblygenerated, therefore it is preferably 0.01 mass % or below.

The balance other than the fundamental components described above is Feand inevitable impurities, but if necessary, it may include one or morekinds selected from a group consist of Cr, Ni, V, Nb, Mo, W, Cu, Ti, Li,Na, K and rare earth metal (REM). Preferable contents when they arecontained are, Cr: 0.5-3 mass %, Ni: 0.5 mass % or below, V: 0.5 mass %or below, Nb: 0.1 mass % or below, Mo: 0.5 mass % or below, W: 0.5 mass% or below, Cu: 0.1 mass % or below, Ti: 0.1 mass % or below, Li: 0.0005mass % or below, Na: 0.0010 mass % or below, K: 0.0010 mass % or below,and REM: 0.0010 mass % or below.

As described above, the spring steel in relation with the secondembodiment of the present invention is excellent in fatigue propertiesbecause the entire inclusions are of low melting point and easy to bedeformed, and SiO₂ is hard to be generated even phase separation occursin heating before hot rolling and during hot rolling. Consequently, thespring steel in relation with this embodiment is useful in manufacturingsprings used, for example, in the automobile field, the industrialmachinery field and the like. In particular, it is best suited tomanufacturing springs used for a restoring mechanism of machinery suchas a valve spring and a clutch spring for an automobile engine, a brakespring, a suspension spring for a suspension and the like which requireextremely high fatigue properties, and the like.

Although the present invention will be described below in further detailby referring to the Examples, the Examples below by no means limit thepresent invention and any design change reflecting the purposesdescribed above and below is to be included within the technical rangeof the present invention.

EXAMPLE 1

Approximately 150 kg of steel and the alloy components other than Si,Al, Mg were molten by a vacuum induction melting furnace, and, in orderto evaluate the degree of hazard of various kinds of oxides, inclusionsin steel were removed (total oxygen: 5 ppm or below) by inductionstirring. Also, in the present example, in order to observe theinfluence of the kind of inclusions on fatigue properties, as shown inTable 1, alloy elements and Fe₂O₃ were added (further, Wallastonite slagwas put on for Nos. 6, 10 only) after deoxidation described above, andmolten steel was prepared so that a variety of inclusions would bepresent.

Then, molten steel was casted in a mold, an ingot obtained was subjectedto forging and hot rolling, and the wire rod with the diameter: 8.0 mmof the componential composition shown in Table 1 was obtained.

Further, if refining is performed in a common method, various kinds ofoxides are spontaneously generated and control of oxide in steel arevery difficult, therefore manufacturing of steel wherein only the targetoxide is present and inclusions of other composition is perfectlyexcluded becomes difficult. As a result, evaluation of the influence onfatigue strength distinguishing each kind of oxide is difficult as well.Consequently it was decided to investigate the influence on fatigueproperties by preparing steel including inclusions of respectivecomposition only by a vacuum induction furnace as described above.

TABLE 1 Experiment Chemical components (mass %)* No. C Si Mn Al Cr Ni VOthers Manufacturing condition 1 0.6 2.0 0.7 0.002 or below 1.8 0.2 0.2— C deoxidation in vacuum → Add alloy elements other than Si → Add Si +Fe₂O₃ 2 0.7 1.5 0.7 0.002 or below 0.6 0.2 — — C deoxidation in vacuum →Add alloy elements other than Si → Add Mg, Si + Fe₂O₃ 3 0.6 2.4 0.50.002 or below — — — — C deoxidation in vacuum → Add alloy elementsother than Si → Add Si + Fe₂O₃ 4 0.6 3.2 0.5 0.002 or below 0.5 — 0.2 —C deoxidation in vacuum → Add alloy elements other than Si → Add Si +Fe₂O₃ 5 0.6 2.0 0.2 0.002 or below — — — — C deoxidation in vacuum → Addalloy elements other than Si → Add Mg, Si + Fe₂O₃ 6 0.7 1.5 1.1 0.002 orbelow 2 0.2 0.8 — C deoxidation in vacuum → Add alloy elements otherthan Si → Add Wallastonite slag → Add Si + Fe₂O₃ 7 0.9 0.5 0.8 0.002 orbelow — — — — C deoxidation in vacuum → Add alloy elements other than Si→ Add Si + Fe₂O₃ 8 0.7 2.1 0.7 0.003 1.5 — 0.4 — C deoxidation in vacuum→ Add alloy elements other than Si → Add Al, Si + Fe₂O₃ 9 0.6 3.2 0.50.002 or below 0.5 — 0.2 Ti: 0.002 C deoxidation in vacuum → Add alloyelements other than Si → Add Si + Fe₂O₃ 10 0.7 1.5 1.1 0.002 or below 20.2 0.8 Mo: 0.007 C deoxidation in vacuum → Add alloy elements otherthan Si → Add Wallastonite slag → Add Si + Fe₂O₃ 11 0.6 2.0 0.7 0.002 orbelow — — 0.2 Li: 0.0001 C deoxidation in vacuum → Add alloy elementsother than Si → Add Si + Fe₂O₃ 12 0.6 2.0 0.7 0.002 or below 1.8 0.2 —Ce: 0.0005 C deoxidation in vacuum → Add alloy elements other than Si →Add Si + Fe₂O₃ 13 0.6 1.5 0.7 0.002 or below 1 0.2 — — C deoxidation invacuum → Add alloy elements other than Si → Add Si + Fe₂O₃ 14 0.6 2.10.6 0.002 or below — 0.2 — — C deoxidation in vacuum → Add alloyelements other than Si → Add Si + Fe₂O₃ 15 0.6 2.5 0.5 0.002 or below —— 0.2 — C deoxidation in vacuum → Add alloy elements other than Si → AddSi + Fe₂O₃ *Balance: iron and inevitable impurities

With respect to each wire rod obtained, measuring of the number ofSiO₂-based inclusions and identification of the kind of coarseinclusions in the microscopic examination plane were performed, andfatigue properties were evaluated. The method of these measurement andevaluation is as described below.

{Measurement of the Number of SiO₂-Based Inclusions}

After the scales on the surface of the wire rod (the diameter: 8.0 mm)were removed, 20 nos. of samples (25 g) were cut out. The samples cutout were molten by the procedure shown in FIG. 2, and inclusions wereabstracted. As exhibited in FIG. 2, aqueous solution of nitric acid 2was put in a beaker 1 first, this aqueous solution of nitric acid washeated (50 DEG C. or above) by a heater [FIG. 2 (a)], and a sample 3 wasput in the heated aqueous solution of nitric acid. The sample 3 wasdissolved by the aqueous solution of nitric acid 2, and the oxide-basedinclusions present in the sample 3 were eluted [FIG. 2 (b)]. The aqueoussolution of nitric acid 2 after finishing dissolution and elution isfiltered (a membrane filter 5, a filter stand 6 and a funnel 7) aftercooling, the aqueous solution of nitric acid 2 is stored in a flask 8,and the oxide-based inclusions were eluted as the residue on themembrane filter 5 [FIG. 2 (c)]. Further, the dissolution described abovewas performed in the manner wherein the nitric acid (HNO₃): 250 mL(milliliter) added with water to make 700 mL and added further with 10mL of sulfuric acid was prepared in a beaker of a 2 L capacity, and asteel sample of 25 g was put in it and was dissolved. Here, the materialof the beaker, funnel and filter stand used is polytetrafluoroethylene[PTFE: Teflon (registered trademark)]. Also, with respect to themembrane filter used in filtering, one with the pore size of 1 μm wasused.

Also, whenever the inclusions described above were abstracted, the blanktest was performed and the abstract in the blank was confirmed to bezero. In the blank test, in performing abstraction and evaluation ofinclusions using the sample, with the beaker without the sample put in,exactly same procedures as the above are followed, with the onlyexception that the sample was not put in, to confirm that the abstractfrom those other than the sample is zero.

Determination of inclusions was performed under the conditions describedbelow. Further, in the quantitative analysis, the presence concentrationof Al, Mn, Si, Mg, Ca, Ti, Zr, K, Na, S, O objecting the inclusions of 4or above of L (the large diameter of an inclusion)/D (the short diameterof an inclusion; the short diameter means the width of the widest linewhich is perpendicular to the long diameter) and 25 μm or above of D andinclusions of less than 4 of L/D and 25 μm or above of L, was obtained.Then, for the oxide-based inclusions with 25% or above of oxygenconcentration, assuming that respective element described above waspresent in the form of Al₂O₃, MnO, SiO₂, MgO, CaO, TiO₂, ZrO₂, K₂O,Na₂O, S, the presence concentration of the oxides and S in theinclusions was calculated on the basis of the concentration ofrespective element obtained by determination described above. Then, thenumber of those with 70% or above of SiO₂ ratio, presumingAl₂O₃+MgO+CaO+SiO₂+MnO=100% (mass %), was obtained, thereafter theaverage value of 20 nos. of samples was obtained to convert to the value(number) per 500 g of steel.

[Analytical Instrument]

-   -   EPMA device: JXA-8621MX made by JEOL    -   Analytical instrument (EDS): TN-5500 made by Tracor Northern

[Analytical Conditions]

-   -   Energy dispersion analysis    -   Voltage (acceleration voltage): 20 kV    -   Test current (acceleration current): 50 nA (nanoampere)    -   Magnification: 150 times    -   Analytical position of an inclusion: SPOT (one point measuring        for the position of the maximum magnitude)

[Photographing Conditions]

-   -   Voltage (acceleration voltage): 20 kV    -   Test current (acceleration current): 50 nA (nanoampere)    -   Photographing magnification: According to the size of an        inclusion

{Measurement of the Composition of Coarse Inclusions in the MicroscopicExamination Plane}

L section including a diameter of respective wire rod described abovewas ground, 1,000 mm² of the surface layer side of D/4 part (D:diameter) of the wire rod was observed, the inclusions with 25 μm orabove width which was perpendicular to the rolling direction wereanalyzed under the conditions described below, and the kind and numberof the inclusions were obtained. Further, MgO-SiO₂ shown in Table 2 arethose with the composition near to MgO-SiO₂ or 2MgO-SiO₂ and are of40-65% SiO₂ content, therefore they are distinguished from theinclusions with stipulated 70% or above of SiO₂ content.

-   -   SPMA device: JXA-8621MX made by JEOL    -   Analytical instrument (EDS): TN-5500 made by Tracor Northern    -   Voltage (acceleration voltage): 20 kV    -   Scanning current: 5 nA (nanoampere)    -   Measuring method: Quantitative analysis by energy dispersion        analysis

[Public Scan (Measurement Over the Entire Particle)]

{Evaluation of Fatigue Properties}

For respective wire rod (8.0 mmφ), after performingscalping→patenting→cold wire drawing working (wire drawing)→oiltempering→process equivalent to strain relieving annealing→shotpeening→strain relieving annealing, 4.0 mmφ×650 mm wires were acquiredas test pieces, a rotating bending test was performed using a Nakamuramethod rotating bending tester under the condition of the testingstress: 908 MPa nominal stress, speed: 4,000-5,000 rpm, number of timesof stoppage: 2×10⁷ times, the number of the test pieces broken due toinclusions (although there were cases wherein the test piece was brokenfrom the surface and the like of the wire not arising from inclusions,such cases were not made the objects) among the test pieces ruptured onthe way was measured, and the breakage ratio was measured according tothe equation below.

Breakage ratio=[number of those broken by inclusion/(number of thosebroken by inclusion+number of those attained the stoppage number oftimes)]×100(%)

The number of SiO₂-based inclusions, the kind and number of coarseinclusions in the microscopic examination face, and fatigue properties(breakage ratio) described above are shown in Table 2.

TABLE 2 SiO₂-based Number of coarse inclusions Breakage ratio inExperiment inclusions in microscopic examination plane fatigue test No.(Nos./500 g) SiO₂ MgO—SiO₂ Wallastonite Anorthite (%) 1 5 0 0 0 0 8 2 70 4 0 0 9 3 6 0 0 0 0 7 4 9 0 0 0 0 7 5 19 0 3 0 0 20 6 11 0 0 2 0 13 714 0 0 0 0 20 8 15 0 0 0 9 18 9 9 0 0 0 0 4 10 11 0 0 3 0 15 11 6 0 0 00 8 12 8 0 0 0 0 6 13 21 0 0 0 0 37 14 25 0 0 0 0 60 15 23 0 0 0 0 44

Based on these results, following consideration is possible (also, theNo. described below shows the experiment No. in the table). It is knownthat the steel of Nos. 1-12 wherein SiO₂-based inclusions are inhibitedas stipulated in the present invention is excellent in fatigueproperties. On the contrary, the steel of Nos. 13-15 wherein SiO₂-basedinclusions exceeds the stipulated region of the present invention isinferior in fatigue properties.

Also, the following is known from Table 2. In observation of themicroscopic examination plane, the observed area is small, and theinclusions such as SiO₂-based inclusions which are present in smallnumbers only are not detected, whereas such inclusions can be alsodetected according to the abstraction method. Further, althoughSiO₂-based inclusions are not detected in the microscopic examinationplane, other crystal-based inclusions are detected, therefore it isknown that other crystal-based inclusions are present more than theSiO₂-based inclusions.

Further, it is known that the breakage ratio correlates not with thenumber of crystal-based inclusions other than SiO₂-based inclusions butwith the number of SiO₂-based inclusions and SiO₂-based inclusions areharmful, and that, consequently, in order to inhibit breakage, it isnecessary to control SiO₂-based inclusions as stipulated in the presentinvention regardless of presence or absence of crystal-based inclusionsother than SiO₂-based inclusions.

EXAMPLE 2

Molten steel which had imitated steel tapped from a converter added witha various kinds of flux was subjected to adjustment of compositions andrefining with slag, and was casted. At this time, the composition ofinclusions was appropriately controlled by appropriately adjusting theslag composition, and steel with the chemical component shown in Table 3below was obtained. The condition of refining with slag for respectivesteel is shown in Table 4 below.

TABLE 3 Test Steel Chemical componential composition (mass %) No. kind CSi Mn Cr Ni V Others 21 A 0.6 2.2 0.5 — 0.2 0.1 — 22 B 0.6 2.0 0.9 1.7 —— — 23 C 0.6 1.4 0.7 — — — — 24 D 0.4 2.8 0.4 — 1.7 0.3 — 25 E 0.6 2.20.5 — 0.2 0.1 Li: 0.0001 26 F 0.6 2.0 0.9 1.7 — — Ce: 0.001 27 G 0.6 1.50.7 — 0.4 — — 28 H 0.6 2.0 0.9 0.9 — 0.1 — 29 I 0.5 1.4 0.7 0.7 — — — 30J 0.6 2.1 0.7 — 1.7 0.2 —

TABLE 4 Chemical componential composition (mass %) Test No. Steel kindAl₂O₃ SiO₂ CaO 21 A 15 42 16 22 B 15 43 35 23 C 24 37 29 24 D 4 46 47 25E 15 42 16 26 F 15 43 35 27 G 30 38 27 28 H 18 54 22 29 I 15 59 20 30 J23 28 47

The molten steel was casted in a mold, the ingot obtained was subjectedto forging and hot rolling, and was made to a wire rod of the diameter:8.0 mm.

For respective hot rolled wire rod obtained, the composition ofoxide-based inclusions in the wire rod was measured and fatigue strengthwas measured. The measuring method of them is as follows.

{Measurement of the Composition of Inclusions}

L section (a section including an axis) of respective hot rolled wirerod was ground, and with respect to 30 nos. of oxide-based inclusions of3 μm or above width (the diameter of an inclusion in the directionperpendicular to the axial direction) present in the surface layer sidefrom the surface of the wire rod to the depth of ¼ of the diameter (thesurface layer side of the position of ½ of the radius from the center ofthe wire rod), analysis of the composition is performed by EPMA, and wasconverted to oxide concentration. The measurement condition of EPMA thenis as follows.

-   -   EPMA device: JXA-8621MX (made by JEOL Ltd.)    -   Analytical instrument (EDS): TN-5500 (made by Tracor Northern        Instruments)    -   Acceleration voltage: 20 kV    -   Scanning current: 5 nA    -   Measuring method: Quantitative analysis by energy dispersion        analysis (measurement over the entire particle)

{Evaluation of Fatigue Properties}

For respective hot rolled wire rod (8.0 mmφ), after performingscalping→patenting→cold wire drawing working (wire drawing)→oiltempering→process equivalent to strain relieving annealing→shotpeening→strain relieving annealing, 4.0 mmφ×650 mm wires were acquiredas test pieces, a test was performed using a Nakamura method rotatingbending tester with the testing stress: 880 MPa nominal stress, speed:4,000-5,000 rpm, the number of times of stoppage: 2×10⁷ times, and therupture ratio was measured according to the equation below for thoseruptured due to inclusions among those which were broken.

Rupture ratio=[number of those broken by inclusion/(number of thosebroken by inclusion+number of those stopped]×100(%)

The composition of inclusions of respective steel is shown in Table 5below. Also fatigue strength (rupture ratio) is shown in Table 6 below.

TABLE 5 Composition of inclusions: Steel 5 elements (mass %) Test No.kind MgO Al₂O₃ SiO₂ CaO MnO Total 21 A 2.3 16.4 42.2 37.4 1.7 100.0 22 B2.0 17.6 43.3 34.7 2.4 100.0 23 C 2.2 23.5 38.1 29.3 1.8 94.9 24 D 1.75.1 45.0 46.7 1.1 99.6 25 E 2.5 15.0 42.3 36.0 1.5 97.3 26 F 1.5 16.540.1 31.0 2.1 91.2 27 G 2.3 31.1 37.9 27.7 0.9 99.9 28 H 2.1 19.1 54.222.7 1.8 99.9 29 I 2.5 15.0 59.0 20.0 3.0 99.5 30 J 1.2 23.0 28.0 47.00.8 100.0

TABLE 6 Result of the fatigue test Test No. Steel kind Rupture ratio (%)21 A 11 22 B 14 23 C 118 24 D 21 25 E 9 26 F 15 27 G 38 28 H 42 29 I 4130 J 35

From these results, the following consideration is possible. It is knownthat, in those of the test Nos. 21-26, the composition of slag isappropriate, the composition of inclusions is controlled to anappropriate region, and excellent fatigue strength is obtained. Amongthem, in those of the test Nos. 21, 22 in particular, the composition ofinclusions is within an optimal region, and excellent fatigue strengthis obtained.

On the other hand, in those of the test Nos. 27-30, because thecomposition of inclusions are deviated from one stipulated in thepresent invention, the result of the fatigue test is not good.

1-9. (canceled)
 10. High cleanliness spring steel, comprising: based onthe total weight of the steel, C: 1.2 mass % or below (not inclusive of0%); Si: 1.2-4 mass %; Mn: 0.1-2.0 mass %; Al: 0.01 mass % or below (notinclusive of 0%), said spring steel comprising oxide-based inclusionswith the width of 3 μm or above present in a surface layer side from asurface of a wire rod to a depth of ¼ of the diameter, wherein thecontent of the oxide-based inclusions satisfies an inequalityrelationship (1) below and the width is defined by a diameter of theinclusion in a direction perpendicular to an axial direction of asection comprising an axial center line of the wire rod; wherein theoxide-based inclusions have: a concentration of MgO of 5 mass % or below(inclusive of 0 mass %) based on the total weight of the inclusion; anda concentration of MnO of 10 mass % or below (inclusive of 0 mass %)based on the total weight of the inclusion, wherein: [Cn], [An] and [Sn]of CaO, Al₂O₃ and SiO₂ in the oxide-based inclusions are respectivelydefined by (2)-(4) below; the concentration of [Cn], [An] and [Sn]satisfy relationships of (5) and (6) below; the melting point of theoxide-based inclusions in a CaO—Al₂O₃—SiO₂ three component system phasediagram, is 1,500 DEG C. or below, and wherein: the concentrations ofMgO, MnO, [Cn], [An], [Sn] are defined by an average value ofoxide-based inclusions in the spring steel,CaO+Al₂O₃+SiO₂>80 (mass %)   (1)[Cn](mass %)=[(CaO)/(CaO+Al₂O₃+SiO₂)]×100   (2)[An](mass %)=[(Al₂O₃)/(CaO+Al₂O₃+SiO₂)]×100   (3)[Sn](mass %)=[(SiO₂)/(CaO+Al₂O₃+SiO₂)]×100   (4)[An]+4.29 [Sn]≦221.9 (mass %)   (5)[An]≦30 (mass %)   (6).
 11. The high cleanliness spring steel as setforth in claim 1 further containing one or more kinds of elementsselected from the group consisting of Cr, Ni, V, Nb, Mo, W, Cu, Ti, Li,Na, K and rare earth elements.
 12. The high cleanliness spring steel asset forth in claim 1, wherein the melting point of the oxide-basedinclusion in a CaO—Al₂O₃—SiO₂ three component system phase diagram is1,400 DEG C. or below.